Method of fabricating silicon-on-insulator pressure detecting device

ABSTRACT

A method of fabricating a silicon structure including forming an insulating layer having an opening on single crystal semiconductor substrate; forming a polycrystalline semiconductor layer on the insulating layer and within the opening in the insulating lating layer; forming an anti-reflective film at spaced apart positions on the polycrystalline semiconductor layer spaced from the opening in the insulating layer by a substantially uniform distance; melting the polycrystalline semiconductor layer by laser irradiation and recrystallizing the polycrystalline semi-conductor layer into a single crystal layer including a quasi-grain boundary; and selectively implanting dopant impurities into the portion of the single crystal layer including the quasi-grain boundary.

This disclosure is a division of Pat. application Ser. No. 08/645,074,filed May 13, 1996 now U.S. Pat. No. 5,677,548.

BACKGROUND OF THE INVENTION

The present invention relates to an SOI (Silicon On Insulator) structurein which a single crystal is grown transversely from a seed crystal onan insulating film, its fabricating method, and a semiconductorpressure-sensing device using the SOI structure.

Conventionally, there has been an SOI structure formed by growing asingle crystal transversely on an insulating film, its fabricatingstructure, and a semiconductor pressure-sensing device using the SOIstructure. One example of making such an SOI structure is a laserrecrystallizing technique in which an amorphous silicon layer formed onan insulating film is irradiated with laser light so that it is meltedand re-solidified (Unexamined Japanese Patent Publication No. 6-112121).The laser recrystallizing technique has an advantage that a high-qualitysingle-crystal film can be formed at a low cost using a manageabledevice, but has a disadvantage that a quasi grain boundary is formedwithin the single crystal.

FIGS. 15 to 18 are sectional views and a sectional perspective view forshowing the conventional SOI structure in which single crystal siliconis formed on an insulating film by a laser recrystallization techniqueand its fabricating method. In FIG. 15, after an insulating layer 2having openings 4 and made of SiO₂ is formed on a single-crystal silicon(Si) substrate 1, a polycrystalline Si (poly-Si) layer 3 is formed onthe surface of the insulating layer 2 and within the openings 4. Itshould be noted that the poly-Si within the opening 4 can be a seedcrystal region. On the surface of the poly-Si layer 3, at positionsspaced from the openings by substantially equal distance,anti-reflective films 5 made of silicon nitride are formed. Thereafter,as shown in FIG. 16, the entire surface is heated by irradiation withlaser light 6. Now, the reflection factor for laser light (e.g.,substantially 0%) of the portions where the anti-reflective films 5 arepresent on the surface of the poly-Si layer 3 is significantly lowerthan that (e.g. about 40%) of the remaining portions where Si isexposed. The poly-Si layer underlying the anti-reflective films 5,therefore, absorbs more laser light and hence is heated to a highertemperature. In other words, the poly-Si within the openings 4 where theanti-reflective films 5 are not formed is maintained at a comparativelylow temperature.

The poly-Si layer 3 melted by irradiation with laser light, cools afterlaser irradiation is ended and, is recrystallized. In this case, asshown in FIG. 17, recrystallization of the molten poly-Si layer 3 isstarted from the portions with a lower temperature. The poly-Si layerwithin the openings 4 is first recrystallized. The recrystallized singlecrystal within the openings 4 resulting from the recrystallization serveas seed portions 41. Recrystallization of the poly-Si layer 3 is startedfrom the seed portions 41. Of the molten poly-Si layer 3, referencenumeral 32 denotes one of a plurality of recrystallized portions, 31denotes one of a plurality of portions in a molten state, and 33 denotesone of a plurality of boundaries between the recrystallized portions 32and the molten portions.

In this way, recrystallization of the poly-Si layer 3 is carried out.Recrystallization is started from the seed portions and advances towardsthe lower portions of the anti-reflective films 5 maintained at a highertemperature. Thus, the crystals grown from both sides toward a centerportion below each of the anti-reflective films 5 collide with eachother. As shown in the sectional perspective view of FIG. 18, a quasigrain boundary 7 sometimes called a subgrain boundary, is generated ateach of colliding portions. These quasi grain boundaries contain manycrystal defects (not shown) which trap carriers (electrons or holes) andconstitute potential barriers. The potential barriers tend to increaseas the impurity concentration in the crystal becomes lower.

FIG. 19 is a sectional view showing the structure of a conventionalsemiconductor pressure-sensing device using the SOI structure thusformed as a pressure-sensing resistor. FIG. 20 is a partial top viewshowing the part of the pressure-sensing resistor of the conventionalsemiconductor pressure-sensing device. In FIGS. 19 and 20, referencenumeral 30 denotes a pressure-sensing resistor in the conventional SOIstructure containing quasi grain boundaries 7. Reference numeral 8denotes an inter-layer insulating film; 9 denotes a wiring layerconnected to the pressure-sensing resistor 30; 10 denotes a protectionfilm covering the inter-layer insulating film and the wiring layer; and11 denotes a diaphragm at the back surface of a silicon (Si) substrate.

The conventional SOI structure formed through the fabrication processdescribed above contains quasi grain boundaries. It is well known thatthe piezo resistance effect increases with a decrease in the impurityconcentration of single crystal Si. In the case of recrystallized Sicontaining quasi grain boundaries, as described above, as the impurityconcentration decreases, the potential barrier generated in the quasigrain boundaries becomes high. The potential barrier also increases withthe crystal defect density at the quasi grain boundary. Since thecrystal defect density itself varies in a wafer, with the impurityconcentration reduced, the piezo resistance effect is greatly influencedby variation in the potential barrier, thus giving rise to anon-uniformity in the resistance of the pressure-sensing resistor withina wafer. Thus, when the impurity concentration is lowered to provide andSOI structure having large piezo resistance effect, the unevenness inthe resistance in the SOI structure within the same wafer increases.Inversely, when the impurity concentration is increased to suppress thenon-uniformity in the resistance, a sufficient piezo resistance effectcannot be obtained.

In a semiconductor pressure-sensing device using the SOI structure as apressure-sensing resistor, it is necessary to compensate for thetemperature of an output in accordance with the operating temperature.In order to compensate for temperature variations with great accuracy,it is necessary to control the resistance of a pressure-sensing resistorand the resistor temperature characteristic with great accuracy.

However, where the conventional SOI structure containing quasi grainboundaries was used as a pressure-sensing resistor of a semiconductorpressure-sensing device, the resistance of the pressure-sensing resistorvaries for each device so that it was difficult to compensate for thetemperature with great accuracy.

An exemplary method of solving such a problem is to formpressure-sensing resistor 35 at a region void of the quasi grainboundaries 7 on the surface of a Si layer 32 formed by recrystallizationas shown in the sectional perspective view of FIG. 21 (JP-A 6-112121).Although this method can reduce non-uniformities in the resistance ofthe pressure-sensing resistor 35, the pressure-sensing resistor 35 mustbe spaced from the quasi grain boundaries 7. This provides a limitationon the size, shape, and location of the pressure-sensing resistor 35 andis an obstacle of designing a device.

SUMMARY OF THE INVENTION

The present invention has been accomplished in order to solve the aboveproblem, and provides an SOI structure with reduced non-uniformities inthe resistance due to quasi grain boundaries without reducing the piezoresistance effect and limiting device designing, its fabricating methodand a semiconductor pressure sensing device which can compensate fortemperature with great accuracy with little variation in the resistanceof a pressure-sensing resistor.

The SOI structure according to the invention comprises a single-crystalsemiconductor substrate, an insulating layer on the single-crystalsemiconductor substrate, a single-crystal semiconductor layer on theinsulating layer and having a quasi grain boundary, and a highly dopedregion having a higher impurity concentration than that of the remainingportion located in a portion including the quasi grain boundary in thesingle-crystal semiconductor layer.

In the SOI structure according to the invention, the impurityconcentration of the high impurity concentration area in thesingle-crystal semiconductor layer is not lower than 4×10¹⁷ cm⁻³.

A method of fabricating an SOI structure according to the presentinvention comprises the steps of: forming an insulating layer having anopening on a single-crystal semiconductor substrate; forming apoly-crystal semiconductor layer on the insulating layer and within theopening of the insulating layer; partially forming an anti-reflectivefilm at positions spaced from the opening of the insulating layer by asubstantially uniform distance on the surface of the insulating layer;melting the poly-crystal semiconductor layer by laser irradiation andcooling to form a single crystal; and selectively implanting impuritiesinto the portion including the quasi grain boundary of thesingle-crystallized semiconductor layer.

Another method of fabricating an SOI structure according to theinvention comprises the steps of: forming an insulating layer having anopening on a single-crystal semiconductor substrate; forming apoly-crystal semiconductor layer on the insulating layer and within theopening of the insulating layer; forming a reflective film having anopening at a position corresponding to the upper portion of the openingof the insulating layer on the surface of the insulating film; meltingthe poly-crystal semiconductor layer by laser irradiation and cooling toform a single crystal; and selectively implanting impurities into theportion including a quasi grain boundary of the single-crystallizedsemiconductor layer using the reflective film as a mask.

A semiconductor pressure-sensing device of the present invention uses,as a pressure-sensing resistor, an SOI structure comprising asingle-crystal semiconductor substrate, an insulating layer on thesingle-crystal semiconductor substrate, and a single-crystalsemiconductor layer on the insulating layer and having a quasi grainboundary, wherein a highly doped region having a higher impurityconcentration than that of the remaining portion is located in a portionincluding the quasi grain boundary in the single-crystal semiconductorlayer.

In the SOI structure according to the present invention, highly dopedregions whose impurity concentration is higher than that of theremaining regions of the recrystallized single crystal semiconductorlayer are formed at portions including quasi grain boundaries.Therefore, unevenness of the resistance is suppressed without reducingthe piezo resistance effect of the SOI structure. This advantage can beassured by setting the impurity concentration of the region not lowerthan 4×10¹⁷ cm⁻³.

The method of fabricating the SOI structure according to the presentinvention comprises a step of selectively implanting impurities intoportions including quasi grain boundaries of the recrystallizedsingle-crystal semiconductor layer. The SOI structure of the presentinvention can be provided by a more simple fabricating process.

The other method of fabricating the SOI structure according to thepresent invention, which uses a reflective film as a selective mask forion-implantation, does not require formation of a photoresist so thatthe quasi grain boundaries are located at the openings, thus providingan advantage of completing the SOI structure by a simpler fabricatingprocess.

In the semiconductor pressure-sensing device according to the presentinvention, a pressure-sensing resistor has an SOI structure with highlydoped regions having an impurity concentration higher than that of theremaining regions of the recrystallized single crystal semiconductorlayer located at portions including a quasi grain boundaries, andprovide an advantage of temperature compensation with great accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional perspective view showing the SOI structureaccording to the first embodiment of the present invention;

FIGS. 2 to 7 are sectional views showing the method of fabricating theSOI structure according to the first embodiment of the presentinvention;

FIGS. 8 and 9 are sectional views showing a semiconductorpressure-sensing device according to the second embodiment of thepresent invention;

FIGS. 10 to 14 are sectional views showing the method of fabricating theSOI structure according to the third embodiment of the presentinvention;

FIG. 15 is a sectional view showing a conventional SOI structure;

FIGS. 16 and 17 are sectional views showing the method of fabricating aconventional SOI structure;

FIG. 18 is a sectional perspective view showing a conventional SOIstructure;

FIG. 19 is a sectional view showing a conventional semiconductorpressure-sensing device;

FIG. 20 is a plan view showing a conventional semiconductorpressure-sensing structure; and

FIG. 21 is a sectional perspective view showing another conventional SOIstructure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Embodiment 1

An explanation will be given of an SOI structure and its fabricatingmethod which are the first embodiment of the present invention. First,referring to sectional views and a sectional perspective view of FIGS. 2to 7, an explanation will be given of a method of fabricating an SOIstructure. It should be noted that the process illustrated in FIGS. 2 to5 is the same as the process of fabricating the conventional SOIstructure. Specifically, in FIG. 2, after an insulating layer 2 havingopenings 4 and made of SiO₂ is formed on a single-crystal silicon (Si)substrate 1, a polycrystalline Si (poly-Si) layer 3 is formed on thesurface of the insulating layer 2 and within the openings 4. It shouldbe noted that poly-Si within the openings 4 can be a seed crystalregion. On the surface of the poly-Si layer 3, at positions spaced fromeach of the openings 4 by a substantially equal distance,anti-reflective films 5 made of a silicon nitride film are formed.Thereafter, as shown in FIG. 3, the entire surface is irradiated withlaser light 6. Now, the reflection factor for laser light (e.g.,substantially zero) of the portions where the anti-reflective films 5are located on the surface of the poly-Si layer 3 is much lower thanthat (e.g. about 40%) of the remaining portions where Si is exposed. Thepoly-Si layer underlying the anti-reflective films 5, therefore,sufficiently absorbs laser light and hence is heated to a highertemperature. In other words, the poly-Si within the openings 4 where theanti-reflective films 5 are not present are maintained at acomparatively low temperature.

The poly-Si layer 31 melted by irradiation with laser light, which coolsafter laser irradiation is ended, is recrystallized. In this case, asshown in FIG. 4, recrystallization of the molten poly-Si layer 3 isstarted from the portions with a lower temperature. The poly-Si layerwithin the openings 4 is first recrystallized. The recrystallized singlecrystal within the openings 4 resulting from the recrystallization serveas seed portions 41. Recrystallization of the poly-Si layer 3 is startedfrom the seed portions 41. Of the molten poly-Si layer 3 in FIG. 4,reference numeral 32 denotes one of a plurality of recrystallizedportions, 31 denotes one of a plurality of portions in a molten state,and 33 denotes one of a plurality of boundaries between therecrystallized portions 32 and the molten portions 31.

In this way, recrystallization of the poly-Si layer 3 is carried out.Recrystallization is started from the seed portions and advances towardsthe lower portions of the anti-reflective films 5 that remain at ahigher temperature. Thus, the crystals grown from both sides toward acenter portion of the lower portion of each of the anti-reflective films5 collide with each other. As shown in the sectional perspective view ofFig. 18, a quasi grain boundary 7 is generated at each of the collidingportions. As described above, the process described hitherto is the sameas the process of the conventional SOI structure.

As seen from FIG. 6, on the surface of the recrystallized Si layer 32, aphotoresist 51 having openings where quasi grain boundaries 52 arelocated is formed. As seen from FIG. 7, using the photoresist 51 as amask, ions are implanted into the portions of the quasi grain boundaries7 of the recrystallized Si layer 32 to form highly doped regions 40having an impurity concentration higher than that of surroundingregions. The recrystallized Si 32, where it is n-type and has animpurity concentration of 1×10¹⁷ cm⁻³ or so, is ion-implanted with,e.g., B⁺ by irradiation with ion beams 61 so that the impurityconcentration of the highly doped impurity regions 40 is 4×10¹⁷ cm⁻³.The photoresist 51 is removed, and annealing is carried out to activatethe impurities of the highly doped regions 40.

The sectional perspective view of the SOI structure thus formed is shownin FIG. 1. In such an SOI structure, the impurity concentration of thehighly doped regions 40, each including a quasi grain boundary 7 of therecrystallized Si layer 32, is higher than that of the surroundingregions. As described previously, since the height of the potentialbarrier is inversely proportional to the impurity concentration, theheight of the potential barrier generated at the quasi grain boundary 7becomes higher than when the highly doped regions 40 are not formed.Thus, non-uniformity in the resistance in the SOI structure due tovariations in the height of the potential barrier can be reduced. On theother hand, because the impurity concentration of the highly dopedregions are enhanced, the piezo resistance effect in these regions islowered. However, since the ratio of the highly doped regions 40 to theremaining regions in the recrystallized Si layer 32 is small (e.g.1/20), the reduction in the piezo resistance effect in the entire SOIstructure is also small. Accordingly, in the SOI structure according tothis embodiment, non-uniformity in the resistance can be suppressedwithout reducing the piezo resistance effect.

The reason why the impurity concentration of the highly doped regions 40is set to 4×10¹⁷ cm⁻³ is based on the experiential fact that the yieldof the semiconductor pressure-sensing device when the above SOIstructure is used as a pressure-sensing resistor is greatly improved atthat impurity concentration or higher.

Embodiment 2

Now referring to FIGS. 8 and 9, an explanation will be given of thestructure of a semiconductor pressure-sensing device which is a secondembodiment of the present invention. FIG. 8 is a sectional view showingthe structure of a semiconductor pressure-sensing device in which theSOI structure according to the first embodiment of the present inventionis used as a pressure-sensing resistor 34. FIG. 9 is a partial top viewshowing the structure of the part of a pressure-sensing resistor 34 ofthe semiconductor pressure-sensing device. In FIGS. 8 and 9, thestructure, other than the pressure-sensing resistor 34, is the same asthat of the conventional semiconductor pressure-sensing device.Specifically, reference numeral 8 denotes an inter-layer insulating film8; 9 denotes wiring layer electrically connected to the pressure-sensingresistor; 10 denotes a protection film covering the inter-layerinsulating film 8 denotes and wiring layer 9; and 11 a diaphragm formedon the back surface of the Si substrate. If the SOI structure accordingto the first embodiment of the present invention is used as apressure-sensing resistor of the semiconductor pressure-sensing device,as described above, a pressure-sensing resistor having little variationin the resistance value can be formed without reducing the piezoresistance effect. Thus, a semiconductor pressure-sensing device withtemperature compensation having great accuracy can be obtained.

Embodiment 3

Referring to the sectional views of FIGS. 10 to 14, an explanation willbe given of a method of fabricating the SOI structure.

An explanation will be given of an SOI structure and a fabricatingmethod according to the first embodiment of the present invention.Specifically, in FIG. 10, after an insulating layer 2 having openings 4and made of SiO₂ is formed on a single-crystal silicon (Si) substrate 1,a polycrystalline Si (poly-Si) layer 3 is formed on the surface of theinsulating layer 2 and within the openings 4. It should be noted thatpoly-Si within the opening 4 can be a seed crystal region. On thesurface of the poly-Si layer 3, at positions spaced from the openings bysubstantially equal distances, reflective films 50 made of a siliconnitride film are formed. Thereafter, as shown in FIG. 11, the entiresurface is irradiated with laser light 6. Now, the reflection factor forlaser light (e.g., 90%) of the portions where the reflective films 50are formed on the surface of the poly-Si layer 3 is significantly higherthan that (e.g. about 40%) of the remaining portions void of thereflective films at openings 501. The poly-Si layer underlying theopenings 501, therefore, absorbs significantly laser light than thepoly-Si layer underlying the reflective films 50 and hence is heated toa higher temperature. In other words, the poly-Si underlying thereflective films 50 of maintained at a comparatively low temperature.

The poly-Si layer 31 melted by irradiation with laser light, cools afterlaser irradiation is ended and, is recrystallized. In this case, asshown in FIG. 12, recrystallization of the molten poly-Si layer 3 startsfrom the portions with a lower temperature. The poly-Si layer within theopenings 4 is first recrystallized. The recrystallized single crystalwithin the openings 4 resulting from the recrystallization serve as seedportions 41. Recrystallization of the poly-Si layer 3 starts from theseed portions 41. In the molten poly-Si layer 3 in FIG. 12, referencenumeral 32 denotes one of a plurality of recrystallized portions, 31denotes one of a plurality of portions in a molten state, and 33 denotesone of a plurality of boundaries between the recrystallized portions 32and the molten portions.

In this way, recrystallization of the poly-Si layer 3 is carried out.Recrystallization is started from the seed portions 41 and advancestowards the lower portions of the openings 501 between the reflectivefilms 5, remains at a higher temperature. Thus, the crystals grown fromboth sides toward a center portion opposite each of openings 501 collidewith each other. As shown in FIG. 13, a quasi grain boundary 7 isgenerated at each of the colliding portions.

As seen from FIG. 14, using the reflective film as a mask, ions areimplanted into the portions of the quasi grain boundaries 7 of therecrystallized Si layer 32 to form highly doped regions 40 with animpurity concentration higher than that of surrounding regions. Therecrystallized Si 32, where it has an n conductivity type and animpurity concentration of 1×10¹⁷ cm⁻³ or so, is ion-implanted with,e.g., B⁺ ion by irradiation with ion beams 61 so that the impurityconcentration of the highly doped impurity regions 40 is 4×10¹⁷ cm⁻³.The reflective films are removed, and annealing is carried out toactivate the impurities of the highly doped regions 40.

As described above, the method of fabricating an SOI structure accordingto the third embodiment is similar to that according to the firstembodiment in that the SOI structure with the non-uniformity ofresistance suppressed, without reducing the piezo resistance effect, canbe obtained by forming the highly doped regions including the quasigrain boundaries 7. However, in the method of fabricating the SOIstructure according to the third embodiment, since the reflective films50 are also used as a mask for ion implantation, unlike the firstembodiment, it is not necessary to form the photoresist 51 so that thegrain quasi grain boundaries 7 are located at the openings 52. Thisembodiment permits the same SOI structure as that according to the firstembodiment to be completed in a simpler fabrication process.

Further, the semiconductor pressure-sensing device in the SOI structureformed by the fabricating method according to the third embodiment isused as a pressure-sensing resistor and has the same advantage asdescribed in connection with the second embodiment.

As described above, in the third embodiment according to the presentinvention, although the laser light 6 was used as means for heating andmelting the Si layer 3, electron beams may be used instead of the laserlight to fabricate an SOI structure according to the present invention.Further, although ion-implantation was carried out using the mask 51 or50 to form the highly doped regions 40, focused ion beams (FIB) may beused to form the same SOI structure without using the mask. The highlydoped regions 40 can also be formed by impurity diffusion. In the firstembodiment, although the photoresist was used as a mask for selectiveion-implantation to form the highly doped regions 40, an insulating filmmade of, e.g., SiO₂, or a poly-Si film may be used as a mask. In theembodiments described above, a silicon nitride film was used as amaterial of the anti-reflective film 5 and the reflective film 50, othermaterials, such as silicon oxide, may be used.

What is claimed is:
 1. A method of fabricating asilicon-insulator-on-insulator structure comprising:forming aninsulating layer having an opening on a single crystal semiconductorsubstrate; forming a polycrystalline semiconductor layer on saidinsulating layer and within the opening in said insulating layer;forming an anti-reflective film at spaced apart positions on saidpolycrystalline semiconductor layer spaced from the opening in saidinsulating layer by a substantially uniform distance; melting saidpolycrystalline semiconductor layer by laser irradiation andrecrystallizing said poly-crystalline semiconductor layer into a singlecrystal layer including a quasi grain boundary; and selectivelyimplanting dopant impurities into the portion of said single crystallayer including the quasi grain boundary.
 2. A method of fabricating asilicon-on-insulator structure comprising:forming an insulating layerhaving an opening on a single crystal semiconductor substrate; forming apolycrystalline semiconductor layer on said insulating layer and withinthe opening in said insulating layer; forming a reflective film on saidpolycrystalline semi-conductor layer and having an opening opposite theopening in said insulating layer; melting said polycrystallinesemiconductor layer by laser irradiation and recrystallizing saidpoly-crystalline semiconductor layer into a single crystal layerincluding a quasi grain boundary; and selectively implanting dopantimpurities into the portion of said single crystal layer including thequasi grain boundary using said reflective film as an ion implantationmask.